Fluke Ti20 user manual Visible, Wavelength µm, Q. What is emissivity?

Ti20

Users Manual

		Visible
Gamma	X-Rays	Ultraviolet Infrared			Radio
Rays					EHF SHF UHF VHF HF MF LF VLF
0.1 A 1 A 1 UA		100 A 0.		1 µ 10 µ 100 µ 0.1 cm 1 cm 10 cm 1 m 10 m 100 m 1 km 10 km 100 km
			1 µ

Wavelength

Infrared Measurement Region

0.4 0.6 0.8 1 1.5 2 3 4 6 8 10 15 20 30

Wavelength µm

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Figure B-1. Infrared Measurement Region

Q. How to assure accurate temperature measurement?

A. A solid understanding of infrared technology and its principles lies behind accurate temperature measurement. When the temperature is measured by a non-contact device the IR energy emitted from the measured object passes through the optical system of the thermometer or thermal imager and is converted to an electrical signal at the detector. This signal is then displayed as a temperature reading and/ or thermal image. There are several important factors that determine accurate measurement. The most important factors are emissivity, distance to spot ratio, and field- of-view.

Q. What is emissivity?

A. All objects reflect, transmit and emit energy. Only the emitted energy indicates the temperature of the object. When IR thermometers or thermal imagers measure the surface temperature they sense all three kinds of energy, therefore all thermometers have to be adjusted to read emitted energy only. Measuring errors are often caused by IR energy being reflected by light sources.

Some IR thermometers and thermal imagers allow you to change the emissivity in the unit. The value of emissivity for various materials can be looked up in published emissivity tables.

Other units have a fixed, pre-set emissivity of 0.95, which is the emissivity value for most organic materials and painted or oxidized surfaces. If you are using a thermometer or thermal imager with a fixed emissivity to measure the surface temperature of a shiny object you can compensate by covering the surface to be measured with masking tape or flat black paint. Allow time for the tape or paint to reach the same temperature as the material underneath. Measure the temperature of the taped or painted surface. That is the true temperature.

B-2